Osteoclasts are required for the normal development of bone during endochondral osification and for the resorption of worn-out bone in the adult skeleton during normal bone remodeling. They also mediate the increased bone loss that occurs in association with inflammation in bone and estrogen deficiency following menopause. Recent studies indicate that expression of M-CSF and RANK (receptor activation of NF-kappaB) ligand is required for osteoclast formation and that activation of genes regulated by the transcription factors, c-fos, PU.1 and NF-kappaB is also necessary. NF-kappaB regulates the expression of the osteoclastogenic cytokines, IL-6, IL-1, and TNF whose expression is up-regulated in inflammatory bone diseases and in response to estrogen deficiency. These cytokines also prevent osteoclast apoptosis, and the increased bone resorption seen after the menopause may in part be due to prolongation of osteoclast life spans on bone surfaces. NF-kappaB has also been shown to prevent TNF- and FAS ligand-induced apoptosis of some cell types and therefore may be involved in the regulation of osteoclast life span. Thus, NF-kappaB may regulate not only the formation of osteoclasts in normal bone remodeling, but also the increased production and prolonged life spans after the menopause. However, the molecular mechanisms whereby NF-kappaB mediates these activities in osteoclasts in osteoclasts are largely unknown and are likely to involve multiple signaling pathways in osteoclasts and their precursors and osteoblasts. We propose to use a combination of in vitro and in vitro approaches to study the role of NF- kappaB in osteoclast formation, activity and survival. Our specific aims are to determine the role of NF-kappaB in 1) osteoclast formation 2) the up- regulation of osteoclastogenesis induced by cytokines and estrogen deficiency and 3) the regulation of osteoclast apoptosis Our underlying hypothesis is that NF-kappaB is required for the activation of genes encoding cytokines which are essential for 1) the progression of osteoclast precursors along a differentiation pathway to form mature osteoclasts; 2) the up-regulation of osteoclastogenesis following estrogen withdrawal; and 3) for the survival of osteoclasts by preventing them from undergoing apoptosis. Understanding the role of NF-kappaB in osteoclastogenesis and survival could lead to the development of new therapeutic agents designed specifically to inhibit bone resorption in conditions, such as postmenopausal osteoporosis, in which it is increased.